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High Voltage Engineering
Introduction
1.1 Generation and transmission of electric energy
The potential benefits of electrical energy supplied to a number
of consumers from a common generating system were
recognized shortly after the development of the ‘dynamo’,
commonly known as the generator.
The first public power station was put into service in 1882 in
London (Holborn). Soon a number of other public supplies for
electricity followed in other developed countries. The early
systems produced direct current at low-voltage, but their
service was limited to highly localized areas and was used
mainly for electric lighting. The limitations of D.C. transmission
at low-voltage became readily apparent. By 1890 the art in the
development of an a.c. generator and transformer had been
perfected to the point when a.c. supply was becoming
common, displacing the earlier D.C. system. The first major
a.c. power station was commissioned in 1890 at Deptford,
supplying power to central London over a distance of 28
miles at 10000 V. From the earliest ‘electricity’ days it was
realized that to make full use of economic generation the
transmission network must be tailored to production with
increased interconnection for pooling of generation in an
integrated system. In addition, the potential development of
hydroelectric power and the need to carry that power over long
distances to the centers of consumption were recognized.
Power transfer for large systems, whether in the context of
interconnection of large systems or bulk transfers, led engineers
invariably to think in terms of high system voltages. Figure 1.1
lists some of the major a.c. transmission systems in
chronological order of their installations, with tentative
projections to the end of this century.
The electric power (P) transmitted on an overhead a.c. line
increases approximately with the surge impedance loading or
the square of the system’s operating voltage. Thus for a
transmission line of surge impedance ZL= 250 ohms at an
operating voltage V, the power transfer capability is
approximately P=V2/ZL , which for an overhead a.c. system
leads to the following results:
The rapidly increasing transmission voltage level in recent
decades is a result of the growing demand for electrical energy,
coupled with the development of large hydroelectric power
stations at sites far remote from centers of industrial activity and
the need to transmit the energy over long distances to the
centers. However, environmental concerns have imposed
limitations on system expansion resulting in the need to better
utilize existing transmission systems. This has led to the
development of Flexible A.C. Transmission Systems (FACTS)
which are based on newly developing high-power electronic
devices such as GTOs and IGBTs. Examples of FACTS systems
include Thyristor Controlled Series Capacitors and
STATCOMS. The FACTS devices improve the utilization of a
transmission system by increasing power transfer capability .
Although the majority of the world’s electric transmission is
carried on a.c. systems, high-voltage direct current (HVDC)
transmission by overhead lines, submarine cables, and back-to-
back installations provides an attractive alternative for bulk
power transfer. HVDC permits a higher power density on a
given right-of-way as compared to a.c. transmission and thus
helps the electric utilities in meeting the environmental
requirements imposed on the transmission of electric power.
HVDC also provides an attractive technical and economic
solution for interconnecting asynchronous a.c. systems and for
bulk power transfer requiring long cables .
1.2 Voltage stresses
Normal operating voltage does not severely stress the power
system’s insulation and only in special circumstances, for
example under pollution conditions, may operating voltages
cause problems to external insulation. Nevertheless, the
operating voltage determines the dimensions of the insulation
which forms part of the generation, transmission and
distribution equipment. The voltage stresses on power systems
arise from various overvoltages. These may be of external or
internal origin. External overvoltages are associated with
lightning discharges and are not dependent on the voltage of the
system. As a result, the importance of stresses produced by
lightning decreases as the operating voltage increases. Internal
overvoltages are generated by changes in the operating
conditions of the system such as switching operations, a fault on
the system or fluctuations in the load or generations.
Their magnitude depends on the rated voltage, the instance at
which a change in operating conditions occurs, the complexity
of the system and so on. Since the change in the system’s
conditions is usually associated with switching operations, these
overvoltages are generally referred to as switching overvoltages.
In designing the system’s insulation the two areas of specific
importance are:
(i) determination of the voltage stresses which the
insulation must withstand, and
(ii) determination of the response of the insulation when
subjected to these voltage stresses.
The balance between the electric stresses on the insulation and
the dielectric strength of this insulation falls within the
framework of insulation coordination and will be discussed in
Chapter 8.
1.3 Testing voltages
Power systems equipment must withstand not only the rated
voltage (Vm), which corresponds to the highest voltage of a
particular system, but also overvoltages. Accordingly, it is
necessary to test h.v. equipment during its development stage
and prior to commissioning. The magnitude and type of test
voltage varies with the rated voltage of a particular apparatus.
The standard methods of measurement of high-voltage and the
basic techniques for application to all types of apparatus for
alternating voltages, direct voltages, switching impulse voltages
and lightning impulse voltages are laid down in the relevant
national and international standards.
1.3.1 Testing with power frequency voltages
To assess the ability of the apparatus’s insulation withstand
under the system’s power frequency voltage the apparatus is
subjected to the 1-minute test under 50 Hz or 60 Hz depending
upon the country. The test voltage is set at a level higher than
the expected working voltage in order to be able to simulate the
stresses likely to be encountered over the years of service. For
indoor installations the equipment tests are carried out under dry
conditions only. For outdoor equipment tests may be required
under conditions of standard rain as prescribed in the
appropriate standards.
1.3.2 Testing with lightning impulse voltages
Lightning strokes terminating on transmission lines will induce
steep rising voltages in the line and set up traveling waves along
the line and may damage the system’s insulation. The magnitude
of these overvoltages may reach several thousand kilovolts,
depending upon the insulation. Exhaustive measurements and
long experience have shown that lightning overvoltages are
characterized by short front duration, ranging from a fraction of
a microsecond to several tens of microseconds and then slowly
decreasing to zero. The standard impulse voltage has been
accepted as a periodic impulse that reaches its peak value in 1.2
.sec and then decreases slowly (in about 50 .sec) to half its peak
value. Full details of the wave shape of the standard impulse
voltage together with the permitted tolerances are presented in
Chapter 2, and the prescribed test procedures are discussed in
Chapter 8.
In addition to testing equipment, impulse voltages are
extensively used in research laboratories in the fundamental
studies of electrical discharge mechanisms, notably when the
time to breakdown is of interest.
1.3.3 Testing with switching impulses
Transient overvoltages accompanying sudden changes in the
state of power systems, e.g. switching operations or faults, are
known as switching impulse voltages. It has become generally
recognized that switching impulse voltages are usually the
dominant factor affecting the design of insulation in h.v. power
systems for rated voltages of about 300 kV and above.
Accordingly, the various international standards recommend
that equipment designed for voltages above 300 kV be tested for
switching impulses. Although the wave-shape of switching
overvoltages occurring in the system may vary widely,
experience has shown that for flashover distances in
atmospheric air of practical interest the lowest withstand values
are obtained with surges with front times between 100 and 300
.sec. Hence, the recommended switching surge voltage has been
designated to have a front time of about 250 sec and half-value
time of 2500 .sec. For GIS (gas-insulated switchgear) on-site
testing, oscillating switching impulse voltages are recommended
for obtaining higher efficiency of the impulse voltage generator
Full details relating to generation, measurements and test
procedures in testing with switching surge voltages will be
found in Chapters 2, 3 and 8.
1.3.4 D.C. voltages
In the past D.C. voltages have been chiefly used for purely
scientific research work. Industrial applications were mainly
limited to testing cables with relatively large capacitance, which
take a very large current when tested with a.c. voltages, and in
testing insulations in which internal discharges may lead to
degradation of the insulation under testing conditions. In recent
years, with the rapidly growing interest in HVDC transmission,
an increasing number of industrial laboratories are being
equipped with sources for producing D.C. high voltages.
Because of the diversity in the application of D.C. high voltages,
ranging from basic physics experiments to industrial
applications, the requirements on the output voltage will vary
accordingly. Detailed description of the various main types of
HVDC generators is given in Chapter 2.
1.3.5 Testing with very low-frequency voltage
In the earlier years when electric power distribution systems
used mainly paper-insulated lead covered cables (PILC) on-site
testing specifications called for tests under d.c. voltages.
Typically the tests were carried out at 4–4.5V0.
The tests helped to isolate defective cables without further
damaging good cable insulation. With the widespread use of
extruded insulation cables of higher dielectric strength, the test
voltage levels were increased to 5–8V0.In the 1970s premature
failures of extruded dielectric cables factory tested under d.c.
voltage at specified levels were noted� �1 . Hence on-site
testing of cables under very low frequency (VLF) of ¾0.1 Hz
has been adopted. The subject has been recently reviewed� 1,2. .
References
1. Working Group 21.09. After-laying tests on high voltage
extruded insulation cable systems,
Electra, No. 173 (1997), pp. 31–41.
2. G.S. Eager et al. High voltage VLF testing of power cables,
IEEE Trans Power Delivery, 12,
No. 2 (1997), pp. 565–570.
2 Generation of high voltages
A fundamental knowledge about generators and circuits which
are in use for the generation of high voltages belongs to the
background of work on h.v. technology.
Generally commercially available h.v. generators are applied in
routine testing laboratories; they are used for testing equipment
such as transformers, bushings, cables, capacitors, switchgear,
etc. The tests should confirm the efficiency and reliability of the
products and therefore the h.v. testing equipment is required to
study the insulation behavior under all conditions which the
apparatus is likely to encounter. The amplitudes and types of the
test voltages, which are always higher than the normal or rated
voltages of the apparatus under test, are in general prescribed by
national or international standards or recommendations, and
therefore there is not much freedom in the selection of the h.v.
testing equipment. Quite often, however, routine testing
laboratories are also used for the development of new products.
Then even higher voltages might be necessary to determine the
factor of safety over the prospective working conditions and to
ensure that the working margin is neither too high nor too low.
Most of the h.v. generator circuits can be changed to increase
the output voltage levels, if the original circuit was properly
designed. Therefore, even the selection of routine testing
equipment should always consider a future extension of the
testing capabilities.
The work carried out in research laboratories varies
considerably from one establishment to another, and the type of
equipment needed varies accordingly.
As there are always some interactions between the h.v.
generating circuits used and the test results, the layout of these
circuits has to be done very carefully.
The classes of tests may differ from the routine tests, and
therefore specially designed circuits are often necessary for such
laboratories. The knowledge about some fundamental circuits
treated in this chapter will also support the development of new
test circuits.
Finally, high voltages are used in many branches of natural
sciences or other technical applications. The generating circuits
are often the same or similar to those treated in the following
sections. It is not the aim, however, of this introductory text to
treat the broad variations of possible circuits, due to space
limitation. Not taken into account are also the differing
problems of electrical power generation and transmission with
high voltages of a.c. or d.c., or the pure testing technique of h.v.
equipment, the procedures of which may be found in relevant
standards of the individual equipment. Power generation and
transmission problems are treated in many modern books, some
of which are listed within the bibliography of an earlier
report.(1)
This chapter discusses the generation of the following main
classes of voltages:
direct voltages, alternating voltages, and transient voltages.
2.1 Direct voltages
In h.v. technology direct voltages are mainly used for pure
scientific research work and for testing equipment related to
HVDC transmission systems. There is still a main application in
tests on HVAC power cables of long length, as the large
capacitance of those cables would take too large a current if
tested with a.c. voltages (see, however, 2.2.2: Series resonant
circuits). Although such d.c. tests on a.c. cables are more
economical and convenient, the validity of this test suffers from
the experimentally obtained stress distribution within the
insulating material, which may considerably be different from
the normal working conditions where the cable is transmitting
power at low-frequency alternating voltages. For the testing of
polyethylene h.v. cables, in use now for some time, d.c. tests are
no longer used, as such tests may not confirm the
�quality of the insulation. 50.
High d.c. voltages are even more extensively used in applied
physics (accelerators, electron microscopy, etc.), electromedical
equipment (X-rays), industrial applications (precipitation and
filtering of exhaust gases in thermal power stations and the
cement industry; electrostatic painting and powder coating, etc.),
or communications electronics (TV, broadcasting stations).
Therefore, the requirements on voltage shape, voltage level, and
current rating, short-or long-term stability for every HVDC
generating system may differ strongly from each other. With the
knowledge of the fundamental generating principles it will be
possible, however, to select proper circuits for a special
application.
The ripple factor is the ratio of the ripple amplitude to the
arithmetic mean value, or .V/V. For test voltages this ripple
factor should not exceed 3 per cent unless otherwise specified
by the appropriate apparatus standard or be necessary for
fundamental investigations.
The d.c. voltages are generally obtained by means of rectifying
circuits
applied to a.c. voltages or by electrostatic generation. A
treatment of the
generation principles according to this subdivision is
appropriate.
2.1.1 A.C. to D.C. conversion
The rectification of alternating currents is the most efficient
means of obtaining
HVDC supplies. Although all circuits in use have been known
for a long time, the cheap production and availability of
manifold solid state rectifiers has facilitated the production and
application of these circuits fundamentally. Since some decades,
there is no longer a need to employ valves, hot cathode gas-
filled valves, mercury pool or corona rectifiers, or even
mechanical rectifiers within the circuits, for which the auxiliary
systems for cathode heating, etc., have always aggravated their
application. The state of the art of such earlier circuits may be
�found in the work of Craggs and Meek, 4. which was written in
1954. All rectifier diodes used now adopt the Si type, and
although the peak reverse voltage is limited to less than about
2500 V, rectifying diode units up to tens and hundreds of kVs
can be made by series connections if
appropriate means are applied to provide equal voltage
distribution during the non-conducting period. One may treat
and simulate, therefore, a rectifier within the circuits –
independently of the voltage levels – simply by the common
symbol for a diode.
The theory of rectifier circuits for low voltages and high power
output is discussed in many standard handbooks. Having the
generation of high d.c. voltages in mind, we will thus restrict the
treatment mainly to single-phase a.c. systems providing a high
ratio of d.c. output to a.c. input voltage. As, however, the power
or d.c. output is always limited by this ratio, and because very
simple rectifier circuits are in use, we will treat only selected
examples of the many available circuits.
3 High Voltage Insulation Engineering
Electrical insulating materials have a very high resistance to the
passage of electrical current under the applied voltage, and
therefore sharply differ in their basic properties from conductive
materials.
In resent year the conductions in which electrical insulating
materials have to operate in electrical engineering fields have
become sever.
Insulator must often survive reliably under high electrical stress
for very long periods of time (fifty year or more). Many
insulating materials may suffer a gradual deterioration; due to
small electrical discharges (scintillations). Insulator breakdown
(BD) can be defined as: "The transition from a stable state of
conduction to a state of high conductivity, for which the
current is largely determined by the source impedance and not
the insulator"
The working voltage of electrical machines, apparatus and all
equipments (such as power under ground cables, over head
transmission lines (OHTL) and transformers) has been
increased. This increase in voltage has caused an essential
increase in the geometrical dimensions of the insulators, the
design of which should be capable to withstand such working
conditions.
In most applications it is desirable to keep the size-cost of an
insulator to a minimum; on the other hand, there are limits to the
electrical stress which can be safely applied to an insulator.
In low voltage deices, increase of insulation thickness in order
to avoid elaborate designs or the need of very extensive
evaluation will often be defensible on economic grounds, which
for high voltage devices the increase in cost with increased
insulator size is often not acceptable if it can be avoided by
means of more elaborate design and more extensive testing.
Cost is not the only argument against increased size (to reduce
electrical stress), the imposition on the environment of ever
larger structure for the transmission, control and distribution of
electrical energy is also highly undesirable. The introduction of
special forms of gaseous insulation has brought about a drastic
change in practices,
Solid insulation does, in general, not recover after electrical
breakdown , i.e. breakdown through the material destroys
permanently its usefulness as insulator, while gaseous insulators
will recover their original strength after removal of the current.
It is thus often desirable to protect the solid insulator from
voltage transients.
Solid Insulator
Breakdown of Solid Insulator
The relation between the breakdown strength of a solid insulator
with respect to time may be given as in the fig below:
Electrical Stress
Electromechanical stress
Thermal stress
kV
μsec Log (time to breakdown) years
Electrochemical stress Tracking Erosion
Electrical strength
The time axis ranges from a fraction of microseconds (us} to
several decades of years.
Several mechanisms can cause a solid insulator to breakdown
and each of them requires different lengths of time to cause
breakdown.
The working stress of equipment must be low enough to prevent
breakdown by any mechanism during the expected life of the
equipment but high enough to ensure manufacturing costs are
economical.
Electrical Breakdown
The term electrical breakdown has several similar but distinctly
different meanings. For example, the term can apply to the
failure of an electric circuit. Alternatively, it may refer to a rapid
reduction in the resistance of an electrical insulator that can
lead to a spark jumping around or through the insulator. This
may be a momentary event (as in an electrostatic discharge), or
may lead to a continuous arc discharge if protective devices fail
to interrupt the current in a high power circuit.
Failure of electrical insulation
The second meaning of the term is more specifically a reference
to the breakdown of the insulation of an electrical wire or other
electrical component. Such breakdown usually results in a short
circuit or a blown fuse. This occurs at the breakdown voltage.
Actual insulation breakdown is more generally found in high-
voltage applications, where it sometimes causes the opening of a
protective circuit breaker. Electrical breakdown is often
associated with the failure of solid or liquid insulating materials
used inside high voltage transformers or capacitors in the
electricity distribution grid. Electrical breakdown can also occur
across the insulators that suspend overhead power lines, within
underground power cables, or lines arcing to nearby branches of
trees. Under sufficient electrical stress, electrical breakdown can
occur within solids, liquids, gases or vacuum. However, the
specific breakdown mechanisms are significantly different for
each, particularly in different kinds of dielectric medium. All
this leads to catastrophic failure of the instruments.
Theoretical Mechanism of Electrical Breakdown
From the bond theory of solid, it is seen that in a perfect crystal
at absolute zero of temperature the bonds are completely filled
up to a certain level and empty thereafter. The upper full on
(valance bond) and the first empty one (conduction bond) are
separated by a forbidden energy gap.
As temperature is increased, electrons may gain sufficient
energy to cross this gap after which they are free to migrate
through the crystal, accelerated by the electric field applied in
their movement.
In practical, solid insulation systems rarely exist on their own; it
is quite likely that solid insulation contains imperfections, i.e.
impurity level exist in the forbidden gap and these can readily
supply electrons into conduction band.
Generally electric breakdown is not of great practical
significance since the operating stress must be smaller than the
electrical breakdown strength in order that the equipments
should survive a useful working life.
Electrical breakdown is characterized by a short time of
development in order of withstand the sudden rise in electric
stress (Impulse Voltage).
Electromechanical Breakdown
When an electric field is applied to a dielectric (insulator)
between two electrodes, a mechanical force will be exerted on
the insulator due the force of attraction between the surface
charges. Therefore as the voltage is increased, the thickness of
the insulator will decrease.
Consider plane electrodes with a solid insulator between them,
let (do) is the initial thickness, then, from general electrostatic
and mechanical strain theories, the field strength at breakdown
(Eb) is given by:
Eb= V/do = 0.6 [Y/ €o€r]1/2
Where:
Y is the Young's Modulus and €o€r is the absolute permittivity
Electro thermal Breakdown
A dielectric to which a voltage is applied liberates heat, the
temperature of the dielectric rises and the losses are therefore
increased still more. The process is intensified until the
dielectric is heated so much that it gets damaged, and the
breakdown of the dielectric occurs at so low voltage at which it
would never develop at a low temperature and in an undamaged
material.
Chemical breakdown
Insulation can degrade chemically even in the absence of an
electric field due to one or more of the following causes:
• Chemical instability
The chemical structure of most insulators breaks down if
subjected to excessive temperature. This causes deterioration of
insulating properties and loss of mechanical strength.
• Oxidation
Some insulation oxidizes in the presence of air. This causes
loss of mechanical strength and cracking the surface of the
insulator. Materials which suffer oxidation include rubbers.
• Polymers in the presence of contamination and prolusion
with humid conditions on its surfaces, small discharges
(scintillations) could occurs on the insulation surfaces,
which can lead to tracking or erosion on the insulation
surface, this may lead to electrical breakdown (tracking) or
mechanical breakdown (erosion).
4 Gaseous Insulation
Introduction
In high voltage apparatus, gases are used for insulation. A gas in
its normal state is an almost perfect insulator. However, when an
electric field (E) of sufficient intensity is established in the gas,
for instante between two electrodes, the gas can become a
conductor, and the transition from insulating to an almost
completely conducing state is called the electrical Breakdown.
The term breakdown is used to describe the flow of electric
current through the insulator gaseous medium.
The requirements for the breakdown case (passage of the
electric current through the gas) are that some of the gas
molecules should be ionized.
Electrical Discharges in Gases
The behavior of a gas under the application of electric field
between two electrodes could in general be studied for the two
following cases;
1. Homogenous field (Uniform field) such field is normally
established using the following electrodes; plane to plane
or sphere to sphere electrodes.
2. Non- homogeneous fields (Non-Uniform field), such field
is normally established using the following electrodes;
plane – rod (point), sphere – rod (point), and rod – rod
(point to point).
Uniform fields
Non Uniform fields